1. Field of the Invention
This invention relates to voltage regulators, and in particular to an improved voltage regulator of the type which provides isolation between an input circuit and an output circuit.
2. Description of Prior Art
My invention, which will be later described, may be employed in a variety of types of voltage regulators. A representative prior art switching type regulator, which serves to describe a common drawback in voltage regulators of the type required to provide isolation between an input circuit and an output circuit, is shown in FIG. la. FIG. 1a shows AC to DC power supply circuit 5, which includes switching regulator circuit 10. Switching regulator circuit 10 provides voltage regulation of DC output voltage V.sub.out while maintaining electrical isolation between the output side and input side of power supply circuit 5. Total electrical isolation between the input and the output is necessary in many high voltage applications for safety reasons to ensure the ground potential at the output is not affected by changes in ground potential at the input. Switching regulator circuit 10 compensates primarily for a change in power supply output load. It does this by monitoring the power supply output voltage V.sub.out. The output voltage on secondary winding W2 of power transformer T1 can be seen reflected on primary winding W1 where it could just as easily be detected and used as a correction signal, however, since there are losses in power transformer T1 and large losses in output rectifier diode D1, it is desireable to monitor the voltage after these loss elements. At 5 volts output, for example, the voltage loss across diode D1 can vary between approximately 0.3 volts to 1.5 volts depending on the current supplied to the output load.
In power supply circuit 5, transistor Q1, in series with primary winding W1 of power transformer T1, acts as a switch, rapidly turning on and off and pulsing DC input voltage V.sub.in through primary winding W1. The switching frequency of transistor Q1 commonly exceeds 20 KHz. Transistor Q1 provides the necessary duty cycle (i.e., time on/time off) of V.sub.in to achieve a desired output voltage V.sub.out, wherein an increased duty cycle results in an increased V.sub.out. FIG. 1b shows the resulting quasisquarewave voltage across winding W1 of power transformer T1 with a peak amplitude of V.sub.in, a period of T.sub.p, and a duty cycle of T.sub.on /T.sub.off.
In power supply circuit 5, when transistor Q1 is on, winding W1, having an inductance L, is charged by the current through the winding, resulting in energy J being stored in the core of power transformer T1, wherein:
ti J=L.multidot.I.sup.2 /2
where
J=energy in joules, PA1 L=inductance in henries of winding W1, and PA1 I=current in amperes through winding W1.
As seen by the opposite dot-indicated polarities of primary winding W1 and secondary winding W2, when transistor Q1 is on and a current flows through winding W1 from the top end of winding W1 to GND1, current flow in winding W2 is from the top end of winding W2 to GND2. And, since the resulting voltage at the anode of output rectifier diode D1, diode D1 being in series with winding W2, is more negative than the voltage, V.sub.out, at its cathode, due to output filtering capacitor C2 being charged to V.sub.out, diode D1 is reverse biased. Thus, energy cannot be transfered through power transformer T1 to the output load connected across filtering capacitor C2. During the time that power transformer T1 is storing energy, filtering capacitor C2, having been previously charged to V.sub.out, as will be described below, supplies the necessary output voltage V.sub.out and current to the output load.
When transistor Q1 is switched off, the voltages across windings W1 and W2 of power transformer T1 reverse polarities due to the magnetic field in the core of transformer T1 trying to collapse. These voltages across windings W1 and W2 rise ntil the voltage across winding W2 forward biases output rectifier diode D1 at a voltage of V.sub.out +V.sub.D1, where V.sub.D1 is the voltage drop across diode D1. The energy stored in power transformer T1 now dissipates into the output load and into capacitor C2 at a voltage, as measured across capacitor C2, equal to V.sub.out and at a current determined by that required to power the output load and to recharge capacitor C2.
Switching regulator circuit 10 uses light emitting diode D2 and phototransistor Q2 to provide isolation between the input and output sections of power supply circuit 5. Switching regulator circuit 10 requires compensation circuits 12 and 14 to compensate for the nonlinearity of the light output of diode D2 with changes in temperature and excitation current, and the nonlinearity of the conductance of phototransistor Q2 with changes in temperature and light input. Potentiometer P1 enables adjustment of the light output of diode D2 for a given V.sub.out. The proper bias current through diode D2 is set by reference voltage V.sub.ref being applied across potentiometer P1, diode D2, and resistor Rl, in series.
Assuming V.sub.out rises above an acceptable level, an increased current, compensated by compensation circuit 12, will be drawn through diode D2, increasing its light output. Phototransistor Q2, detecting an increased light input, conducts more current, thus increasing the voltage drop across collector resistor R2, and hence, lowering the input voltage into compensation circuit 14. Compensation circuit 14 provides a corresponding lower voltage into regulator control circuit 16, which, if regulator control circuit 16 is a pulsewidth modulator, provides a quasisquarewave input voltage, corresponding to the duty cycle of voltage V.sub.in needed to produce the desired V.sub.out, into the control terminal of switching transistor Q1. Thus, as seen, power supply circuit 5 provides a relatively constant V.sub.out while maintaining electrical isolation between the output and input sections of power supply circuit 5 via the optical coupling. A schematic diagram of an actual regulated power supply using opto-isolation is shown in the book, "Power MOSFET Transistor Data", by Motorola, Inc., FIGS. 7-3 and 7-4, page A-74, 1984, and is herein incorporated by reference.
This method of providing isolation is common and works well. The drawbacks are mostly cost and complexity, partially due to the cost of the opto-isolator and partially due to the compensation circuits necessary to compensate for the nonlinearity of diode D2 and transistor Q2. Also, since most opto-isolators have a broad manufacturing tolerance, a variable potentiometer is commonly installed, which must be adjusted in the production stage. This adds parts and labor costs and decreases reliability, since unsealed variable potentiometers lend themselves to failure as a result of vibration or environmental conditions.